JP2001047006A - System and method for removal of dioxins in ash - Google Patents

Abstract

PROBLEM TO BE SOLVED: To decompose dioxins contained in ashes after waste combustion in air. SOLUTION: Combustion ashes are heated to 551 deg.C or higher in air to decompose dioxins contained in the combustion ashes in a fluidized bed heating furnace and gaseous dioxins which are incompletely decomposed by the heating decomposition and discharged together with heated gas are decomposed by electron beam to remove dioxins.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a system for removing dioxins contained in ash after burning waste.

[0002]

2. Description of the Related Art As waste treatment, volume reduction treatment by combustion is generally performed. However, in the combustion process,
Dioxins (2,3,3), which are highly poisonous harmful substances, are generated by the combustion of organochlorine compounds contained in waste.
7,8-tetrachlorodibenzo-p-dioxin and its derivatives, hereinafter referred to as “dioxin”),
It is a problem worldwide. In Japan, regulations on the emission of dioxin in exhaust gas have been enacted, but dioxin in ash, which is said to contain most of the generated dioxin, has not yet been enforced. It seems likely that dioxins in ash will be regulated in the future.

[0003] Fig. 4 is a graph showing the relationship between the dioxin decomposition rate and the dioxin concentration with respect to fly ash temperature in the presence of oxygen. Is shown in FIG. 5 (see “Thermal decomposition treatment of dioxins in fly ash”, Nikkei Technical Report Vol. 55, pp. 27-33). From the graph of FIG. 4, it can be seen that the decomposition rate of dioxin in ash, that is, the rate of thermal decomposition for dechlorination and hydrogenation, increases exponentially with an increase in fly ash temperature. Further, in a state of lack of oxygen, even if the ash is heated in a temperature range of 200 to 550 ° C., generation of dioxins does not occur, and a dramatic decrease in dioxin concentration has been observed. The mechanism of this dioxin decomposition is considered to be mainly dechlorination and hydrogenation by the catalytic action of fly ash, and the conditions for obtaining a high thermal decomposition rate of dioxin in ash are as follows:
(1) In a state of lack of oxygen, (2) Ash at 200 to 550 ° C
(See "Dioxin decomposition treatment in fly ash" above).

FIG. 5 shows a thermal decomposition apparatus based on this concept by the thermal dechlorination method. According to the flow of the pyrolysis apparatus, first, the ash collected by the bag filter 1 of the waste combustion plant is temporarily stored in the fly ash hopper 3 via the conveyor 2 and then passed through the rotary valve 4 to the reactor 25. Sent to Here, the ash is heated by the electric heater 24 while being stirred and transferred, and after reaching the set temperature, is kept at a constant temperature, and the dioxins in the ash are thermally decomposed. On the other hand, the steam and the like generated in the reactor 25
5 guided to the dust filter 26 attached to the upper part,
The steam is condensed by the condenser 27 and the accompanying inert gas such as N ₂ gas for sealing the shaft of the stirrer is supplied to the recirculation fan 28.
To the reactor 25. The fly ash discharged from the reactor 25 is cooled by the cooler 29, stored in the bunker 9 via the discharge rotary valve 30, and a series of steps of thermal decomposition of dioxin in the ash is completed. In this device,
It has been reported that dioxin in ash of 99% or more is decomposed when the reactor temperature is 350 to 400 ° C and O ₂ is ₂ to 3% or less, and the residence time is 1 hour or more.

[0005]

However, in this apparatus, the atmosphere for ash heating needs to be in an oxygen-deficient state, and the use of an inert gas such as N ₂ gas and the construction of an apparatus for preventing air from entering. Is required, and the system is complicated. Also, just stirring the ash in the reactor,
It is difficult to uniformly heat the ash, and a residence time of one hour or more is required for the thermal decomposition of dioxin, so that there has been a problem that the reactor vessel becomes large.
Furthermore, it is difficult to completely decompose by thermal decomposition, and there is a problem that about 0.1 to several% is transferred to the gas side and remains without being decomposed. Accordingly, an object of the present invention is to provide an apparatus and a method for performing high-efficiency decomposition of dioxin in ash in air.

[0006]

DISCLOSURE OF THE INVENTION The present invention provides a method for producing ash that is at least above the maximum temperature of 550 ° C. in the prior art.
A fluidized bed heating furnace for heating and decomposing dioxin contained in the combustion ash by air heating to 51 ° C. or higher, and a dioxin gasified without being completely decomposed contained in the heating gas generated from the heating furnace Provided is a highly efficient system for removing dioxin in ash, comprising a humidifier and an electron beam, which are means for decomposing ash.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The dioxin removal system in ash of the present invention will be described with reference to FIG. 1 as an example. The removal system of the present invention heats combustion ash generated in a garbage incineration plant or the like,
A fluidized bed heating furnace for thermally decomposing dioxins contained in the combustion ash, and a humidifier and an electron beam irradiation device as means for decomposing gasified dioxins in the heated gas that could not be completely decomposed in the fluidized bed furnace .

In FIG. 1, from the upstream to the downstream, a bag filter 1, a conveyor 2, a hopper 3, a fluidized bed heating furnace 5, a humidifier 11, an electron beam irradiation device 12, a bag filter 16
Are arranged in this order. The gas processed by the bag filter 16 is discharged from the chimney 18 by the attraction fan 17. The fluidized bed heating furnace 5 is further provided with an auxiliary burner 15,
The FDF 13 and the air heater 1 are provided upstream of the auxiliary burner 15.
4 are arranged in order. To remove dioxin, first, room temperature air is introduced from the FDF 13 and the air heater 14 is removed.
After preheating to 300 ° C with the auxiliary fuel burner 15, 700 ° C
The heating is performed as described above, and the mixture is guided into the fluidized bed heating furnace 5 to be a fluidized bed heating gas. Ash collected by the bag filter 1 of a waste combustion plant such as a garbage incineration plant is stored in the hopper 3 via the conveyor 2. The ash stored in the hopper 3 is introduced into the fluidized bed heating furnace 5 by opening the rotary valve 4, mixed with the fluidized bed heating gas, and heated to 551 ° C. or higher. The heated ash stays in the fluidized bed for one hour or more, passes through the equally-divided partition plate 7, is discharged from the discharge port 19 in an overflow manner, and is discharged through the rotary valve 8 through the bunker 9.
Is stored in

[0009] The temperature of dioxin in the ash heated in the fluidized bed is 551 ° C or higher. FIG. 6 shows the concentration of dioxins (PCDFs, PCDDs) after the ash was heated in nitrogen and air for one hour. FIG. 6 is a diagram showing “generation and dechlorination of dioxins in ESP ash from municipal solid waste incineration facilities” (Safety Engineering, Vol. 32, No. 1 (1)
993), page 14, by Tamkun et al.). From FIG. 6, it can be seen that at 300 ° C. or higher in nitrogen, the concentration of dioxins is reduced by progress of decomposition. On the other hand, in air, 300-350 ° C
In the range above, an increase in dioxins due to so-called resynthesis is observed, but at 450 ° C. or higher, a decrease due to decomposition of dioxins is observed almost as in nitrogen.
Since this data is one sample of ash, it is not necessarily 45
If it is 0 ° C., it does not indicate that it is equivalent to that in nitrogen. Therefore, the heating temperature is set to 551 ° C., which is more than 550 ° C. of the prior art, to surely promote the decomposition.

In another document, dioxins are reduced at 400 ° C. or higher in air as in the above-mentioned document, but even at 600 ° C., a stable PC is not obtained.
It has been reported that DB (polydibenzofuran) migrates to the trace gas side (H. Vogg and L. St.).
ielitz "THERMAL BEHAVIORO
F PCDD / PCDF IN FLYASH FRO
M MUNICIPAL INCLINEATOR
S ", Chempshare, 15, 1373-137
8 (1986)). The trace amount of dioxin transferred to the gas side is decomposed by a humidifier and an electron beam irradiation device. As shown in FIG. 1, after the fly ash from the fluidized bed is separated by the cyclone 10 installed between the fluidized bed heating furnace and the humidifier, water is added through the humidifier 11. After the addition of water, the mixture is guided to the electron beam irradiation devices 12 arranged in multiple stages (in the drawing, two stages as an example), where the dioxin in the gas is decomposed by the electron beam irradiation devices. The electron beam irradiation device 12 has one or several stages arranged in series after the humidifier 11. From the efficiency of dioxin irradiation, it is desirable to arrange several stages of electron beam irradiation devices 12 in series.

FIG. 2 is a schematic view of the electron beam irradiation device 12. As shown in FIG. In this system, the fluidized bed heating gas containing dioxin transferred to the gas side is separated from fly ash by the cyclone 10 and then introduced into the electron beam irradiation device 12.
Preferably, a gas containing dioxin heated to 551 ° C. or higher by a fluidized bed heating furnace is sprayed with water by a humidifier 11 and further irradiated with an electron beam by an electron beam irradiation device, so that chemicals such as OH radicals are produced. An active component is generated, and this component acts on dioxin to decompose dioxin.

The electron beam irradiation apparatus shown in FIG. 2 mainly comprises a power supply 401, an electron gun 402, a deflection coil 403, an electron beam irradiation window 404, and a reaction vessel 405. The deflection coil 403 is connected to the electron gun 402 and the power supply 401, respectively. The deflection coil 403 is directed substantially perpendicularly to the reaction vessel 405, and electrons emitted from the deflection coil 403 are emitted almost perpendicularly to the electron beam irradiation window 404. The reaction vessel 405 includes a part of a line through which the gas humidified by the humidifier 11 passes, and the gas is irradiated with electrons at a vertical angle through the electron beam irradiation window 404.

The gas that has exited the humidifier is led to the reaction vessel 405 of the electron beam irradiation device 12. At that time, 100 ke
Electrons having high energy of V or more are extracted by the electron gun 402, deflected by the deflection coil 403, and driven into the reaction vessel 405 through the electron beam irradiation window 404. When H ₂ O or O ₂ in the gas collides with the injected electrons, highly reactive oxidizing radicals such as OH radicals are generated. This oxidizing radical oxidizes and decomposes dioxin, whereby the dioxin is decomposed.

It has been reported that 85 to 90% or more of dioxin is decomposed at an electron beam power of 10 kGy ('Electron Beam Induced Decomposi').
tion of Chlorinated Aromatic Compounds in Waste In
cinerator Offgas', H.-R.Paur, et al. 10th Interna
tional Meeting on Radiation Processing (IMRP 10), M
ay 12-16, 1997, Anaheim (USA)). For example, if 99% of dioxin in ash is decomposed in a fluidized bed, the remaining 1% moves to the gas side. It is assumed that 90% of this 1% dioxin is decomposed by irradiating an electron beam. Then, the total dioxin decomposition efficiency is 99.9%
Becomes Therefore, electron beam irradiation with a power of about 5 to 15 kGy is performed. In the humidifier 11, if an alkaline agent (regardless of the form of slurry or powder) is sprayed at the same time as moisture, HCl generated by the decomposition of dioxin can be neutralized and HCl can be removed. Examples of alkaline agents that can be used include NaO
H, KOH and the like. The alkali agent does not necessarily need to be sprayed simultaneously with the water, but may be sprayed in the form of an aqueous solution of the alkali agent in a humidifier, or may be sprayed in any line between the cyclone and the bag filter.

In FIG. 1, two stages of electron beam irradiation devices 12 are installed in series to increase the efficiency of decomposition. The gas in which dioxin is decomposed by the electron beam is supplied to the air heater 14.
After the temperature is reduced to about 170 ° C., the remaining fly ash generated in the fluidized bed heating furnace by the bag filter 16 is removed. The gas that finally decomposes and removes dioxin and fly ash is
After the fine ash is collected by the bag filter 16, the fine ash is discharged from the chimney 18 via the attraction fan 17. The ash collected by the bag filter is returned to the supply hopper 3 again by a device (not shown).

In the fluidized bed heating furnace of the example shown in FIG. 1, since the amount of heating gas is large and the amount of ash scattered is large, a vibrating fluidized bed may be used to improve the amount. FIG. 3 shows an example. The vibrating fluidized bed has a spring 32 between the heating furnace 5 and the fixed base 31, and a vibration motor 3 installed in the heating furnace 5 body.
Due to 3, the heating furnace body vibrates. The ash in the heating furnace is fluidized by this vibration. To heat the ash, the electric heater 34 is inserted directly into the ash deposition portion of the heating furnace main body, and the ash is heated to 551 ° C. or higher by the electric heater. In this vibrating fluidized bed, since air is not used for fluidization, ash scattered in the generated heating gas is extremely small. Therefore, the gas system without the cyclone and the bag filter in FIG. 1 is used.

[0017]

According to the above-mentioned system, dioxin in ash in the air can be decomposed in the fluidized bed heating furnace, and there is an advantage that the heating furnace can be downsized and no inert gas or the like is used. Further, by decomposing gasified dioxin, which cannot be completely decomposed in the fluidized bed heating furnace, by the electron beam irradiation device, an effect of making the emission of dioxin infinitely “zero” can be obtained.

[Brief description of the drawings]

FIG. 1 shows a system diagram of a system for removing dioxin in ash of the present invention.

FIG. 2 shows an electron beam irradiation device that can be used in the removal system of the present invention.

FIG. 3 is a system diagram of the system of the present invention of FIG. 1 when the fluidized bed is a vibrating fluidized bed.

FIG. 4 is a graph showing a relationship between dioxin concentration and decomposition rate with respect to fly ash temperature.

FIG. 5 is a conventional dioxin decomposer by thermal decomposition by a thermal dechlorination method.

FIG. 6 is a graph showing a change in thermal decomposition of dioxin in ash when heated for 1 hour in a nitrogen atmosphere or an air atmosphere, and shows “generation and desorption of dioxins in ESP ash of municipal solid waste incineration facility. Chlorination ”(Safety Engineering, V
ol32, No. 1 (1993), p. 14) is partially modified.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 16 Bag filter 2 Conveyor 3 Hopper 4, 8 Rotary valve 5 Fluidized bed heating furnace 6 Diffusion plate 7 Equally divided partition plate 9 Bunker 10 Cyclone 11 Humidifier 12 Electron beam irradiation device 13 FDF 14 Air heater 15 Fuel auxiliary burner 17 Attraction Fan 18 chimney 19 discharge port 24, 34 electric heater 25 reactor 26 dust filter 27 condenser 28 recirculation fan 29 cooler 30 discharge rotary valve 31 fixed base 32 spring 33 vibration motor 401 power supply 402 electron gun 403 deflection coil 404 electron beam irradiation window 405 reaction vessel

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Morio Kagami 1-1-1, Wadazakicho, Hyogo-ku, Kobe-shi, Hyogo F-term in Kobe Shipyard, Mitsubishi Heavy Industries, Ltd. (reference) 2E191 BA13 BD11 BD17 3K061 BA07 EA05 EB14 NA01 4D004 AA36 AB07 CA24 CA43 CA50 CB08 DA03 DA06

Claims (3)

[Claims] 1. A fluidized bed heating furnace for heating combustion ash to 551 ° C. or higher in air to decompose dioxin contained in the combustion ash, and a decomposition bed contained in gas discharged from the heating furnace. Means for decomposing dioxin gasified without being treated. 2. The means for decomposing gasified dioxin is a humidifier and an electron beam irradiation device.
2. The system for removing dioxin in ash according to item 1. 3. a step of heating the combustion ash to 551 ° C. or higher by air heating to decompose dioxin contained in the combustion ash, and a step of decomposing dioxin gasified without being partially decomposed by the heat decomposition. For removing dioxins in ash containing